1. Field of the Invention
The present invention relates to a rotary anode type X-ray tube and, more particularly, an improvement of the rotary mechanism in the rotary anode type X-ray tube.
2. Description of the Related Art
As is known, in a rotary-anode type X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft which have a bearing portion therebetween, and an electron beam emitted from a cathode is radiated on the anode target while the anode target is rotated at a high speed by energizing an electromagnetic coil arranged outside a vacuum envelope, thus irradiating X-rays. The bearing portion is constituted by a roller bearing, such as a ball bearing, or a hydrodynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium-indium-tin (Ga-In-Sn) alloy, which is liquified during an operation. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-113817, 60-117531, and 2-244545.
In the rotary anode type X-ray tube provided with one of the hydrodynamic pressure type slide bearings disclosed in the above-mentioned gazettes and in which the liquid metal lubricant is used, a clearance between the rotary and the stationary structures is inevitably opened and communicated with a space in a vacuum envelope so that the liquid metal lubricant may be leaked into the space in the vacuum envelope through the opening. When the liquid metal lubricant is leaked from the bearing section, it becomes insufficient in volume in the bearing section. Thus, stable dynamic pressure type bearing operation cannot be maintained for a long time. When the liquid metal lubricant is scattered into the space in the vacuum envelope, the withstanding voltage property of the X-ray tube is remarkably damaged.
In order to prevent these damages, the abovementioned Japanese Patent Disclosure No. 2-244545 discloses that rare metal film which is made wet with the lubricant to form a mixed phase or alloy is formed on those surfaces of stationary and rotary structures, which are faced to each other. However, in this arrangement the liquid metal lubricant leaked from the bearing section is concentrated on corners of a clearance between the stationary and the rotary structures by centrifugal force, and deposited thereon and alloyed therewith. When the liquid metal lubricant is accumulated at the corners of the clearance in this manner, the clearance is made smaller and smaller by the lubricant thus accumulated, thereby causing the rotor cylinder not to be rotated. Further, the clearance between the stationary and the rotary structures is shaped like a simple L when sectioned in half and this makes it easy for the liquid metal lubricant to be leaked and scattered into the space in the vacuum container, passing through the clearance.